1. Field of the Invention
The present invention relates to polyisocyanates which are blocked with CH-acidic esters and stabilized with formaldehyde against thermal yellowing, to a process for their preparation and to their use in one-component coating compositions.
2. Description of the Prior Art
The use of malonic esters or malonates as blocking agents for isocyanates is known and disclosed, e.g., in Liebigs An. Chem. 562 (1949) p. 205 et seq. A specific, more recent variant of this type of blocking is described, for example, in EP-A 600,314. The advantages of polyisocyanate crosslinking agents blocked with CH-acid esters is that crosslinking temperatures of about 100.degree. C. are achieved and these blocking agents are easy to handle on a large industrial scale.
Blocked polyisocyanates with low crosslinking temperatures of about 100.degree. C. would represent an attractive industrial alternative to melamineformaldehyde resins if they did not tend to yellow due to the presence of the blocking agent.
It is known that one-component polyurethane stoving lacquers containing polyisocyanates, which are blocked with malonic esters and/or acetoacetic esters, can severely yellow when cured, especially when they are overstoved. It is known from DE-A 19 637 334 that the yellowing can be reduced or prevented by the addition to blocked isocyanates of hydrazides and HALS amines which are unsubstituted on the N atom. However, these blocked isocyanates cannot be employed for all car painting applications.
An object of the invention is to provide polyisocyanates which are blocked with CH-acid esters and show no tendency towards thermal yellowing.
This object can be achieved with the stabilized, blocked polyisocyanates according to the present invention which contain formaldehyde.